1. Field of the Invention
The invention relates to polyester resins and more particularly to a method of branching linear polyester resins.
2. Brief Description of the Prior Art
Linear polyesters such as, for example, poly (ethylene terephthalate) (referred to hereafter at times as "PET") and poly (1,4-butylene terephthalate) (referred to hereafter at times as "PBT") are widely known and used to fabricate a variety of thermoplastic articles; see for example U.S. Pat. No. 4,684,686. Many of their properties, including chemical stability and solvent resistance make them attractive candidates for such forming operations as blow molding, profile extrusion and thermoforming. One problem in such operations is the relatively low melt viscosities of the polyesters, as a result of which the formed articles do not always adequately retain their shape immediately after forming and before they have cooled.
One method of increasing the melt viscosity of a linear polyester, described in U.S. Pat. No. 4,590,259, is to substantially increase its molecular weight. This is possible (though often difficult, owing to the crystallinity of the polyester) if the polyester has a low proportion of carboxylic acid end groups. However, many polyesters currently available have a relatively high proportion of such groups, which makes molecular weight increase impractical as a means of increasing melt viscosity. In any event, specialized equipment is usually required to increase a polyester's molecular weight.
Even if the polyester molecular weight is high, melt behavior often leaves something to be desired since it is essentially independent of shearing stresses imposed during forming. In the best situation, a resin suitable for blow molding, profile extrusion, thermoforming and the like will exhibit a non-Newtonian response, having a low viscosity under high shear conditions such as those encountered during extrusion and a high viscosity under conditions of low shear typical of a blow molded parison or a thermoformed article. High molecular weight polyesters under melt conditions have the disadvantage of being essentially Newtonian in their behavior.
The present invention achieves thermomolding compositions exhibiting higher melt strengths by a branching of the polyester, which can take place during the forming process, for example during extrusion.
An advantage of the present invention it that it does not require specially designed facilities for polyester production. Conventional processing equipment may be employed for the branching of conventional polyesters such as PET, PBT and like elastomeric polyesters.